The long term goal of these studies is to define the innate host-virus interactions that occur during hepatitis C virus (HCV) infection with the hope that this will lead to novel therapeutic approaches to this disease. HCV is a noncytopathic, positive-strand RNA virus that causes acute and chronic hepatitis and hepatocellular carcinoma. Approximately 2-4 million persons are chronically infected by HCV in the USA and 170 million people are chronically infected worldwide, many of whom will die from liver failure and hepatocellular carcinoma. The recent development of a robust cell culture model of HCV infection permits analysis of the entire HCV life cycle and facilitates analysis of the host-virus interactions that determine the outcome of HCV infection. Using that model we recently reported that HCV doesn't induce type 1 interferon or interferon stimulated genes in infected cells; that the viral NS3/4A protease blocks double stranded (ds) RNA signaling by cleaving a key intermediate in the dsRNA signaling pathway; and that HCV evades the innate host response in infected cells by an entirely novel NS3/4A-independent mechanism that is mediated by the HCV NS4B protein. We also discovered that type 1 interferon rapidly modulates the expression of a subset of cellular microRNAs (miRNAs) that partially mediate the antiviral effects of IFN against HCV. These findings identify a previously unsuspected effector arm of the interferon response that contributes to the control of HCV infection. In the current proposal we will extend these studies with the following Specific Aims. In Specific Aim 1, we will characterize the molecular mechanism whereby NS4B blocks double stranded RNA-signaling by determining if it directly interacts with RIG-I, if it regulates RIG-I subcellular localization, and if it blocks the ability of RIG-I to bind dsRNA. In Specific Aim 2, we will identify the repertoire of cellular miRNAs that mediate the antiviral effect of interferon against HCV, identify the cellular genes that mediate their antiviral effects, and characterize the mechanism whereby those genes control HCV infection. Collectively, these experiments will provide insight into viral evasion and host defense mechanisms that could lead to the development of novel antiviral strategies to terminate chronic HCV infection.